Conditions on the course are getting softer as we head into winter and a ball will often plug after a stroke then hop out of its plug mark before coming to rest. If this plug mark is near the ball you cannot repair it before you make a stroke as in doing so you are improving the conditions affecting the stroke by which is meant improving the area of intended stance or swing, lie of the ball and line of play so resist the temptation to press it down with your foot! By all means repair the plug mark afterwards. The rules do allow us to repair plug marks on the green and it is good etiquette to repair any marks you see whether or not you were responsible for making them either before or after you putt.